chore(docs): Update dev install instructions

[chendaniely]

Fixes #1720

pip install https://github.com/posit-dev/py-shiny/tarball/main
was causing errors during install.

The error message mentions:

      For example, if you're using pip, instead of https://github.com/user/proj/archive/master.zip use git+https://github.com/user/proj.git#egg=proj

running pip install git+https://github.com/posit-dev/py-shiny.git seem to work

see #1720 for more details.